There has long been a need to employ catalysts in reactions such as simultaneous oxidation of carbon monoxide and unburned hydrocarbons, and the reduction of nitrogen oxides, NOx, (three-way catalysis) which are emitted from automotive engines and the like. The role of catalysts, particularly three-way catalysts, in automotive emission control has been widely studied in the art. For example, Taylor, "Automobile Catalytic Converter", Catalysis, Science and Technology, pp. 119-67 (Anderson et al. eds. 1984), describes emission control technology, composition of three-way catalysts, and catalytic supports.
Conventional systems for converting automotive exhaust gases employ a pre-fabricated supported catalyst, typically a solid stratum of catalyst material, such as honeycombed ceramic structures, which are placed in the exhaust section of the automobile. As the emissions pass through the solid, the catalytic metal present on the strata aids in conversion of CO, NOx and unburned hydrocarbons to CO.sub.2, N.sub.2 and H.sub.2 O. However, the solid strata-type catalytic converter is eventually expended and require removal and replacement in the exhaust portion of the engine. Moreover, structures such as a honeycomb support are complex and relatively expensive to manufacture. State of the art systems capable of carrying out three-way catalysis include those having supported noble metals such as rhodium and platinum, with rhodium being a preferred catalyst for the reaction: EQU NO+CO.fwdarw.1/2 N.sub.2 +CO.sub.2
Platinum is the preferred catalyst for oxidation of CO and unburned hydrocarbons.
The noble metals, particularly rhodium, are expensive and in limited supply. This situation is exacerbated by the fact that current usage of rhodium (Rh) in three-way catalysis exceeds the mine ratio of Rh/Pt. Thus, reduction of noble metal usage is necessary for three-way catalysis processes. Therefore, it is desirable to develop alternative approaches to emission control.
In particular, there is a need for alternative economical methods of converting automotive emissions not utilizing conventional non-regenerable solid catalytic material-containing supports in the exhaust system of an automobile.
In an attempt to meet this need, attempts have been made to develop ways to improve fuel combustion and/or to abate the exhaust gases. For example, U.S. Pat. No. 4,891,050 describes gasoline additives comprising platinum group metal compounds which are said to improve operating efficiency of internal combustion engines, in terms of power output per unit of fuel burned, and which are said to reduce the emissions of particulates and noxious gases, such as hydrocarbons and carbon monoxide. Reduction of NOx is also referred to in the reference, but is not supported by any data disclosed in the reference. The disclosed catalytic metal compounds are initially dissolved in an organic solvent miscible in gasoline. All tested compounds in the reference are organometallic compounds containing ligands with unsaturated C--C bonds. The reference does not appear to teach or suggest any catalytic effect occurring outside the combustion chamber.
U.S. Pat. Nos. 4,295,816, 4,382,017 and 4,475,483 describe catalyst solutions and delivery systems for improving the efficiency of combustion chambers. The catalyst solutions described in U.S. Pat. No. 4,382,017 comprise a single metal catalyst compound, H.sub.2 PtCl.sub.6.6H.sub.2 O; a chloride compound such as HCl, LiCl, or NaCl; an antifreeze compound such as ethylene glycol; and approximately 50 percent water by volume. The chloride is a blocking agent which prevents precipitation and destruction of the platinum compound which, it is said, would otherwise occur by use of the antifreeze compound. The solutions are not taught or suggested for use in aiding conversion of automotive emissions, require the chloride "blocking agent," and contain undesirably high levels of water.
U.S. Pat. No. 4,295,816 describes a catalyst delivery system including a single platinum group metal catalyst in water. A layer of oil containing a manganese catalyst is provided on top of the surface of the water. Air is bubbled through the water and is said to meter minute amounts of catalyst to a combustion system, where the catalyst is consumed in the combustion reaction. The patent does not teach or suggest that the solution could be used for deposition onto a surface within the exhaust system of an automobile. The patent does not teach or suggest conversion of emissions from combustion chambers.
U.S. Pat. No. 4,475,483 describes a catalyst delivery system similar to that described in U.S. Pat. No. 4,295,816, with a single rhenium metal catalyst used in place of a platinum group metal catalyst in the water. The patent further describes that an antifreeze agent such as a glycol, dissolves the water along with the catalyst. The patent teaches that if an antifreeze agent is employed, a blocking agent such as NaCl, HCl, or LiCl must be employed to prevent precipitation of the catalyst. The patent does not teach or suggest conversion of emissions from a combustion chamber.
Thus, it can be seen that these known systems involve the use of catalytic solutions or suspensions which are delivered directly to the fuel or are disposed in the combustion air stream. However, there are disadvantages associated with the use of catalytic solutions. First, the solutions themselves may be detrimental to the combustion process or the emission abatement process. Furthermore, the cost of preparing the solutions represents an expense over and above the cost of a conventional solid catalyst and support. For example, in accordance with U.S. Pat. No. 4,382,017 the catalytic solution includes a blocking agent consisting of HCl and LiCl, which are highly corrosive substances. This patent further describes a solution of ethylene glycol and water as the solvent in which to dissolve the metals, thereby wasting costly glycol and introducing an inhibitor (i.e., water) to the combustion environment.
In the prior art, it has not been possible to effectively deliver catalytic additives directly to fuels without solvents or other extraneous agents.